The present invention relates to triangular-pyramidal cube-corner retroreflective elements having a novel structure and a triangular-pyramidal cc retroreflective assembly in which the triangular-pyramidal cube-corner retroreflective elements are arranged. More minutely, the present invention relates to retroreflective elements (hereafter simply referred to as retroreflective elements or reflective elements) such as triangular-pyramidal cube-corner retroreflective elements constituting a retroreflective body useful for reflectors such as signs including traffic signs and construction work signs, visible tapes of vehicles and motorcycles, safety materials of clothing and life preservers, markings of signboards, and reflectors of visible light, laser beam, or infrared-ray reflective sensors, and an assembly of the retroreflective elements.
Still more minutely, the present invention relates to triangular-pyramidal cube-corner retroreflective elements characterized in that triangular-pyramidal cube-corner retroreflective elements protruding beyond a common bottom plane (Sx-Sxxe2x80x2) share one base edge (x) on the bottom plane (Sx-Sxxe2x80x2) and are arranged in the closest-packed state so as to be faced each other, the bottom plane (Sx-Sxxe2x80x2) is a common plane including a plurality of the base edges (x,x, . . . ) shared by the triangular-pyramidal reflective elements, two triangular-pyramidal reflective elements faced each other include the shared base edges (x,x, . . . ) on the bottom plane (Sx-Sxxe2x80x2) and form a substantially same-shape element pair faced each other so as to be symmetric to planes (Lxxe2x80x94Lx, Lxxe2x80x94Lx, . . . ) vertical to the bottom plane (Sx-Sxxe2x80x2), and when assuming the height from the bottom plane (Sx-Sxxe2x80x2) including the base edges (x,x, . . . ) shared by the two triangular-pyramidal reflective elements faced each other up to apexes (H1 and H2) of the triangular-pyramidal reflective elements as hx, the height from a bottom plane (Sy-Syxe2x80x2) including other base edges (y,y, . . . ) of the triangular-pyramidal reflective elements up to the apexes (H1 and H2) of the triangular-pyramidal reflective elements as hy, and the height from a bottom plane (Sz-Szxe2x80x2) including still other base edges (z,z, . . . ) of the triangular-pyramidal reflective elements up to the apexes (H1 and H2) of the triangular-pyramidal reflective elements as hz, at least any two of hx, hy, and hz are substantially different from each other and a mirror reflective layer is formed on reflective side faces of the triangular-pyramidal reflective elements.
A retroreflective body for reflecting entrance light toward a light source has been well known so far and the reflective body using its retroreflectivity is widely used in the above industrial fields. Particularly, a triangular-pyramidal cube-corner retroreflective body (hereafter also referred to as a CC reflective body) using the internal-total-reflection theory such as triangular-pyramidal cube-corner retroreflective elements (hereafter also simply referred to as triangular-pyramidal reflective elements or CC reflective elements) are remarkably superior to a retroreflective body using conventional micro glass beads in retroreflective efficiency of light and thereby, uses of the triangular-pyramidal cube-corner retroreflective elements have been increased year by year because of its superior retroreflective performance.
However, though conventional publicly-known triangular-pyramidal retroreflective elements show a preferable retroreflective efficiency when an angle formed between the optical axis (axis passing through the apex of a triangle equally separate from three faces constituting triangular-pyramidal cube-corner retroreflective elements and inserting with each other at an angle of 90xc2x0) of the elements and an entrance ray is small because of the reflection theory of the elements, the retroreflective efficiency is suddenly lowered (that is, the entrance angle characteristic is deteriorated) as the entrance angle increases. Moreover, the light entering the face of the triangular-pyramidal reflective elements at an angle less than the critical angle (xcex1c) meeting an internal-total-reflection condition decided in accordance with the refractive index of a transparent medium constituting the triangular-pyramidal reflective elements and that of air reaches the back of the elements without totally reflecting from the interface of the elements. Therefore, a retroreflective sheeting using triangular-pyramidal reflective elements generally has a disadvantage that it is inferior in entrance angularity.
However, because triangular-pyramidal retroreflective elements can reflect light in the direction in which the light enters over the almost entire surface of the elements, reflected light does not reflect by diverging at a wide angle due to spherical aberration like the case of micro-glass bead reflective elements. However, the narrow divergent angle of the reflected light easily causes a trouble that when the light emitted from a head lamp of an automobile retroreflects from a traffic sign, it does not easily reach, for example, eyes of a driver present at a position separate from the optical axis of the head lamp. The frequency of the above type of the trouble increases more and more (that is, observation angularity is deteriorated) because an angle (observation angle) formed between the entrance axis of rays and the axis connecting a driver with a reflection point increases.
Many proposals have been made so far for the above cube-corner retroreflective sheeting, particularly for a triangular-pyramidal cube-corner retroreflective sheeting and various improvements are studied.
For example, the specification of Jungersen""s U.S. Pat. No. 2,481,757 discloses retroreflective elements assembly in which all base edges of triangular-pyramidal reflective elements are present on the same plane, the optical axis of each retroreflective elements tilts from the direction vertical to the basic plane, and a mirror reflective layer is formed on surfaces of prism side faces of the retroreflective elements. These retroreflective elements form a retroreflective element pair faced each other at the both sides of a shared base edge and the optical axis of the retroreflective element pair tilts in directions opposite to each other.
Moreover, the official gazette of Stamm""s Japanese Patent Laid-Open No. Sho 49-106839 (specification of U.S. Pat. No. 3,712,706) discloses a retroreflective sheeting constituted by an assembly of normal triangular-pyramidal cube-corner retroreflective elements each of whose bottom planes is an equilateral triangle and on each of whose reflection-side surfaces a mirror reflective layer is formed. The optical axis of each of the retroreflective elements is vertical to the bottom plane of the elements.
The triangular-pyramidal cube-corner retroreflective elements in the above two proposals respectively have a mirror reflective layer on surfaces of their prism side faces. Therefore, incoming light hardly passes through retroreflective elements but it is substantially entirely reflected. Therefore, when comparing the above retroreflective elements with triangular-pyramidal cube-corner retroreflective elements having no mirror reflective layer according to only the internal total reflection theory, all rays to be retroreflected greatly increase and are superior in entrance angularity.
However, CC reflective elements designed so that the optical axis tilts have a problem that differences between areas of three reflective side faces (faces a, b, and c) excessively increase and the retroreflective performance is deteriorated.
The present inventor et al. have found in recent years that it is possible to improve the entrance angularity of a retroreflective sheeting constituted by the above retroreflective elements by substantially making the depth (hxe2x80x2)[equal to the height of apexes (H1 and H2) from the bottom plane (X-Xxe2x80x2)] of the elements of a face (face c) having a base edge on the bottom plane (X-Xxe2x80x2) of the retroreflective elements substantially larger than the depth (h) of a face (virtual face Z-Zxe2x80x2) including base edges (z, and w) of two faces substantially perpendicularly crossing with the face c of the triangular-pyramidal reflective elements from the apexes. The invention of the present inventor et al. is disclosed in the official gazette of U.S. Pat. No. 2,954,709.
However, also in the case of these elements, the trouble is not solved that when the entrance angle of entrance rays increases, the internal total-reflection condition is not satisfied and the entrance light does not reflect on the reflection side face but it passes through elements.
It is an object of the present invention to provide triangular-pyramidal cube-corner retroreflective elements (CC reflective elements) superior in retroreflective performance at an entrance angle of 30xc2x0 or more, preferably at an angle of 40xc2x0 or more.
According to the present invention, the above object and advantage are achieved by triangular-pyramidal cube-corner retroreflective elements characterized in that the triangular-pyramidal cube-corner retroreflective elements protruding beyond a common bottom plane (Sx-Sxxe2x80x2) share one base edge (x) on the bottom plane (Sx-Sxxe2x80x2) and are arranged in the closest-packed state so as to be faced each other, the bottom plane (Sx-Sxxe2x80x2) is a common plane including a plurality of the base edges (x,x, . . . ) shared by the triangular-pyramidal reflective elements, two triangular-pyramidal reflective elements faced each other include the shared base edges (x,x, . . . ) on the bottom plane (Sx-Sxxe2x80x2) and form a substantially same-shape element pair faced each other so as to be symmetric to planes (Lxxe2x80x94Lx, Lxxe2x80x94Lx, . . . ) vertical to the bottom plane (Sx-Sxxe2x80x2), and when assuming the height from the bottom plane (Sx-Sxxe2x80x2) including the base edges (x,x, . . . ) shared by the two triangular-pyramidal reflective elements faced each other up to apexes (H1 and H2) of the triangular-pyramidal reflective elements as hx, the height from a bottom plane (Sy-Syxe2x80x2) including other base edges (y,y, . . . ) of the triangular-pyramidal reflective elements up to the apexes (H1 and H2) of the triangular-pyramidal reflective elements as hy, and the height from a bottom plane (Sz-Szxe2x80x2) including still other base edges (z,z, . . . ) of the triangular-pyramidal reflective elements up to the apexes (H1 and H2) of the triangular-pyramidal reflective elements as hz, at least any two of hx, hy, and hz are substantially different from each other and a mirror reflective layer is formed on reflective side faces of the triangular-pyramidal reflective elements.
In the case of the present invention, triangular-pyramidal cube-corner retroreflective elements are preferable in which the optical axis of the triangular-pyramidal reflective elements tilts so that the angle formed between the optical axis and a vertical line extended from apexes (H1 and H2) of the elements to the bottom plane (Sx-Sxxe2x80x2) of the optical axis ranges between 0.5xc2x0 and 12xc2x0 in the direction for the difference (qxe2x88x92p) between the distance (q) from the intersection (Q) of the optical axis and the bottom plane (Sx-Sxxe2x80x2) up to base edges (x,x, . . . ) shared by the element pair and the distance (p) from the intersection (P) of the vertical line and the bottom plane Sx-Sxxe2x80x2) up to the base edges (x,x, . . . ) shared by the element pair to become plus (+) or minus (xe2x88x92).
In the case of the present invention, triangular-pyramidal cube-corner retroreflective elements are preferable in which the optical axis of the triangular-pyramidal reflective elements tilts so that the angle formed between the optical axis and the above vertical line ranges between 0.5xc2x0 and 1.5xc2x0.
In the case of the present invention, triangular-pyramidal cube-corner retroreflective elements are also preferable in which the optical axis of the triangular-pyramidal reflective elements tilts so that the angle formed between the optical axis and the above vertical line ranges between 4xc2x0 and 12xc2x0.
More preferable triangular-pyramidal cube-corner retroreflective elements of the present invention are triangular-pyramidal cube-corner retroreflective elements in which the optical axis of the triangular-pyramidal reflective elements tilts in the direction for the difference (qxe2x88x92p) between the distance (q) from the intersection (Q) of the optical axis and the bottom plane (Sx-Sxxe2x80x2) up to base edges (x,x, . . . ) shared by the element pair and the distance (p) from the intersection (P) of a vertical line extended from apexes (H1 and H2) of the elements to the bottom plane (Sx-Sxxe2x80x2) and the bottom plane (Sx-Sxxe2x80x2) up to the base edges (x,x, . . . ) shared by the element pair to become plus (+) and hx is substantially larger than hy and hz.
Still more preferable triangular-pyramidal cube-corner retroreflective elements of the present invention are triangular cube-corner retroreflective elements in which the above hy and hz are substantially equal to each other and hx is substantially larger than hy and hz.
Still more preferable triangular-pyramidal cube-corner retroreflective elements of the present invention are triangular-pyramidal cube-corner retroreflective elements in which the optical axis of the triangular-pyramidal reflective elements tilts in the direction for the difference (qxe2x88x92p) between the distance (q) from the intersection (Q) of the optical axis and the bottom plane (Sx-Sxxe2x80x2) up to base edges (x,x, . . . ) shared by the element pair and the distance (p) from the intersection (P) of a vertical line extended from apexes (H1 and H2) of the elements to the bottom plane (Sxxe2x80x94Sx) and the bottom plane (Sx-Sxxe2x80x2) up to base edges (x,x, . . . ) shared by the element pair to become minus (xe2x88x92) and hx is substantially smaller than hy and hz.
Still more preferable triangular-pyramidal cube-corner retroreflective elements of the present invention are triangular-pyramidal cube-corner retroreflective elements in which the above hy and hz are substantially equal to each other and the above hx is substantially smaller than the above hy and hz.
Still more preferable triangular-pyramidal cube-corner retroreflective elements of the present invention are triangular-pyramidal cube-corner retroreflective elements in which when assuming the maximum one of the above hx, hy, and hz as hmax and the minimum one of them as hmin, the following expression is effectuated.
1.03 less than hmax/hmin less than 1.3 
Still more preferable triangular-pyramidal cube-corner retroreflective elements of the present invention are triangular-pyramidal cube-corner retroreflective elements in which the above hx, hy, and hz respectively range between 50 and 500 xcexcm (both included).
The present invention further provides a triangular-pyramidal cube-corner retroreflective sheeting in which the above two triangular-pyramidal cube-corner retroreflective elements faced each other are arranged in the closest-packed state to form a sheeting while sharing base edges (x,x, . . . ).
The present invention is described below more minutely.